1. Field of Invention
The present invention relates to the technical field of an electrooptic device such as a liquid-crystal device, and more particularly, to a thin-film transistor (hereinafter referred as TFT) and an active-matrix liquid-crystal display device, which adopts an alternating drive method in which the polarities of the voltages applied to adjacent pixels are periodically alternated in every pixel row or every pixel column so that the voltages applied to adjacent pixels in a row direction or in a column direction are inverted in polarity.
2. Description of Related Art
The electrooptic device of this sort typically adopts an alternating drive method in which the polarity of a voltage applied to the pixel electrodes is alternated at a predetermined pattern to prevent degradation of the electrooptic material, as a result of the application of a direct current, and to control a cross-talk and flickering of a display screen image. A 1H alternating drive method is relatively easy to control and presents a high-quality image display, wherein during the presentation of a video signal of one frame or one field, the pixel electrodes arranged on an odd row are driven by a voltage that is positive relative to the potential of a counter electrode, while the pixel electrodes arranged on an even row are driven by a voltage that is negative relative to the potential of the counter electrode, and during the presentation of a video signal of a next frame or a next field, conversely, the pixel electrodes arranged on the even row are driven by a positive voltage while the pixel electrodes arranged on the odd row are driven by a negative voltage (in other words, the pixel electrodes on the same row are driven by the same polarity voltage and the voltage polarity is alternated every row with the period of frame or field).
A 1S alternating drive method is also easy to control and presents a high-quality image display, wherein the pixel electrodes on the same column are driven by the same polarity voltage, while the voltage polarity is alternated every column with the period of frame or field.
Further, a dot alternating drive method has been developed which periodically reverses the polarity of the voltage applied to each pixel electrode from pixel electrode to pixel electrode in the direction of columns or in the direction of rows.
When the voltages of the adjacent pixel electrodes in a TFT array substrate (i.e., the voltages applied to the pixel electrodes adjacent in the column direction in the 1H alternating drive method, the voltages applied to the pixel electrodes adjacent in the row direction in the 1S alternating drive method, and the voltages applied to the pixel electrodes adjacent in the row direction and the column direction in the dot alternating drive method) are opposite in polarity as in the above-referenced 1H alternating drive method, the 1S alternating drive method, and the dot alternating drive method, a transverse electric field (specifically, an electric field in parallel with the surface of the substrate or an slant electric field having a component in parallel with the surface of the substrate) takes place between the adjacent pixel electrodes. If such a transverse electric field is applied to the electrooptic material, which is expected to work under a longitudinal electric field present between the pixel electrodes and the counter electrode (i.e., an electric field perpendicular to the surface of the substrate), an orientation defect takes place in the electrooptic material, an unlit defect occurs there, and the contrast ratio drops. Although the area of the transverse electric field can be covered with the light shield layer, the aperture of the pixel is reduced with the size of the area of the transverse electric field. As the distance between the adjacent pixel electrodes is reduced with a fine pixel pitch, the transverse electric field intensifies, and these become more problematic as high-definition design increases in the electrooptic device.
The present invention has been developed in view of the above problems, and it is an object of the present invention to provide a method for manufacturing an electrooptical device and the electrooptical device which reduces a malfunction due to the transverse electric field in the electrooptical material, such as a liquid crystal, while presenting a high-contrast, bright and high-quality image.
To achieve the above object, in accordance with the invention, a method for manufacturing an electrooptical device, which includes a first substrate, a second substrate, and an electrooptical material interposed between the first and second substrates, the first substrate including a plurality of two-dimensionally arranged pixel electrodes, including pixel electrodes in a first group driven in a periodic polarity reversal manner with a first period and pixel electrodes in a second group driven in a periodic polarity reversal manner with a second period which is complementary to the first period, and the second substrate including a counter electrode arranged to face the plurality of pixel electrodes, includes a step of forming a pattern including a wiring that drives the pixel electrodes, and elements on the first substrate, a step of planarizing the top surface of the laminate on the first substrate including the pattern, a step of forming a protrusion in an area in a spacing between pixel electrodes adjacent in a plan view, by subjecting the planarized surface to photolithographic and etching processes, and a step of fabricating the plurality of pixel electrodes.
The electrooptical device manufactured in accordance with the manufacturing method of the present invention includes, on the first substrate, the plurality of two-dimensionally arranged pixel electrodes, including the first group pixel electrodes driven in the periodic polarity reversal manner with the first period and the second group pixel electrodes driven in the periodic polarity reversal manner with the second period which is complementary to the first period, and the second substrate includes a counter electrode arranged to face the plurality of pixel electrodes. Therefore, the first substrate includes (i) adjacent pixel electrodes that are respectively driven by mutually opposite polarity voltages during the periodic polarity reversal driving, and (ii) adjacent pixel electrodes that are respectively driven by the same polarity voltages during the periodic polarity reversal driving. The two types of pixel electrodes are present in the electrooptic device, such as a matrix-type liquid-crystal display device, as long as it is driven in the above-referenced 1H alternating drive method or 1S alternating drive method. The transverse electric field takes place between the adjacent pixel electrodes belonging to the different pixel electrode groups (i.e., the adjacent pixel electrodes supplied with the opposite polarity voltages).
In accordance with the present invention, the planarizing step planarizes the top surface of the laminate on the first substrate (i.e., the top surface of an insulator to be planarized formed on top of an irregular surface having a wiring on an interlayer insulator) including the pattern of the wiring for driving the pixel electrodes (such as data lines, scanning lines, and capacitive lines) and elements (such as pixel switching TFTs). The step of forming the protrusion in the area in the spacing between the pixel electrodes adjacent in a plan view is accomplished by subjecting the planarized surface to photolithographic and etching processes in succession. The pixel electrodes are then formed.
Regardless of the wiring and the elements formed below, the surface underlying the pixel electrodes includes the positively planarized area having no protrusion, and the area having the protrusion which is positively elevated to a predetermined height in a predetermined configuration. As a result, the central portion of each pixel electrode centered in the aperture of each pixel is formed on the positively planarized surface. The electrooptical device is thus free from malfunction of the electrooptical material, such as the orientation defect of the liquid crystal arising from variations in the thickness of the electrooptical material encapsulated between the pixel electrode and the counter electrode.
First, since the protrusion is positively formed in the area between the adjacent pixel electrodes through an etching process, the longitudinal electric field taking place between the pixel electrode and the counter electrode is relatively intensified with respect to the transverse electric field taking place between the adjacent pixel electrodes (in particular, the pixel electrodes belonging to the different pixel electrode groups) if the edge of each pixel electrode is formed to be positioned on the top of the protrusion. Generally, the shorter the distance between the electrodes, the stronger the electric field therebetween. Since the edge of the pixel electrode is positioned closer to the counter electrode by the height of the protrusion, the longitudinal electric field taking place therebetween accordingly intensifies. Second, regardless of whether the edge of the pixel electrode is positioned on the top of the protrusion, the transverse electric field taking place between the adjacent pixel electrodes (in particular, the pixel electrodes belonging to the different pixel electrode groups) is weakened by the dielectric constant of the protrusion, and further, the influence of the transverse electric field on the electrooptical material is reduced by reducing the volume of the electrooptical material through which the transverse electric field passes (with the electrooptical material partly replaced in volume with the protrusion). The electrooptical device is thus free from malfunction of the electrooptical material, such as the orientation defect of the liquid crystal arising from the transverse electric field caused in any of the alternating drive methods. In this case, the edge of the pixel electrode may reach the top of the protrusion as discussed above, or may not reach the top of the protrusion. Alternatively, the edge of the pixel electrode may extend to a midway point on the slant wall of the protrusion or on the substantially vertically upright wall of the protrusion.
The protrusion is much more precisely controlled with regard to the height thereof and the configuration thereof, than in the technique in which the height of the pixel electrode at the edge thereof is adjusted using the presence of the wiring and the elements present beneath the underlying surface of the pixel electrodes (a combination of unavoidable slight misalignments between patterns of numerous layers makes it fundamentally difficult to form a protrusion and a recess on the final top layer at designed height and configuration). The electrooptical device thus provides high reliability, free from the malfunction of the electrooptical material, such as the orientation defect of the liquid crystal arising from the transverse electric field.
Compared with the case in which variations exist in the thickness of the electrooptical material attributed to the presence of the wirings and the elements beneath the underlying surface of the pixel electrodes, the use of the positively planarized surface results in substantially small thickness variations. The electrooptical device is thus free from the malfunction of the electrooptical material such as the orientation defect of the liquid crystal arising from the variations in the thickness of the electrooptical material.
Further, since a light shield layer that covers the malfunctioning point of the electrooptical material is reduced in size, visible defects, such as unlit defects, are prevented, and the aperture ratio of each pixel electrode is increased.
As a result, malfunction due to the transverse electric field in the electrooptical material, such as a liquid crystal is substantially reduced, and an electrooptical device, such as a liquid-crystal device, presenting a high-contrast, bright and high-quality image, is relatively easily manufactured.
The protrusion may be formed in only the area between the adjacent pixel electrodes belonging to the different pixel electrode groups (i.e., the adjacent pixel electrodes supplied with the opposite polarity voltages) where the transverse electric field takes place. It is not necessary to form the protrusion in the area between the adjacent pixel electrodes belonging to the same pixel electrode group where no substantial transverse electric field takes place. Even if the protrusion is arranged at that location, the above-described advantage of the present invention may be provided by arranging a relatively low-profile protrusion.
The above-referenced technique is particularly effective when an element is manufactured of an active layer of a monocrystal semiconductor to enhance the performance of the driver element. The monocrystal semiconductor layer is formed of a monocrystal layer on a support substrate typically using a layer stacking method. Since the support substrate and the monocrystal layer surface are planarized and mirror-surfaced before attaching them in the layer stacking method, controlling flexibly protrusions and recesses subsequent to the formation of the elements and the wirings is difficult. As discussed above, however, configuration control is facilitated by planarizing the underlying surface beneath the pixel electrodes and by forming the protrusion. This prevents the liquid-crystal orientation defects.
In one embodiment of the manufacturing method of the electrooptical device of the present invention, the planarizing step may include depositing an insulator having a predetermined thickness, and forming a planarized insulator by subjecting the insulator having the predetermined thickness to a CMP (Chemical Mechanical Polishing) process.
In this embodiment, in the planarizing step, the insulator having the predetermined thickness is formed, and is then subjected to the CMP process. The planarized insulator thus results. The height and shape of the protrusion are easily controlled with a high accuracy.
In another embodiment of the manufacturing method of the electrooptical device of the present invention, the planarizing step may include forming a planarized insulator by applying a flowable insulator material.
In accordance with this embodiment, the planarized insulator is formed by applying the flowable insulator material through a spin coating technique or the like. In this planarizing step, the height and shape of the protrusion are relatively easily controlled with a high accuracy.
In yet another embodiment of the manufacturing method of the electrooptical device of the present invention, an element that drives the pixel electrode formed on the first substrate may be fabricated of a monocrystal semiconductor layer based on a layer stacking SOI (Silicon On Insulator) technique. The protrusion of the pixel electrode is thus flexibly formed with a good controllability through the planarization step subsequent to the formation of the element.
In still yet another embodiment of the manufacturing method of the electrooptical device of the present invention, the planarizing step may include forming beforehand a groove into which the pattern is embedded.
In accordance with this embodiment, in the planarizing step, the groove is formed in the interlayer insulator in or on the first substrate prior to the formation of the pattern including the wiring and the elements. At least the pattern is then partly embedded into the groove. The surface beneath the protrusion is relatively easily planarized. As a result, the height and shape of the protrusion are relatively easily controlled with a high accuracy.
In still yet another embodiment of the manufacturing method of the electrooptical device of the present invention, the step of forming the protrusion may form the protrusion in a grid configuration running in the spacing between the adjacent pixel electrodes.
In this embodiment, the step of forming the protrusion forms the protrusion in the grid configuration running in the spacing between the adjacent pixel electrodes. The protrusion grid thus reduces the adverse effect of the transverse electric field during the periodic polarity reversal driving regardless of whether the adjacent pixel electrodes belonging to the different pixel electrode groups are arranged in a direction of rows or a direction of columns within an image display area.
In this embodiment, the step of forming the protrusion may form the protrusion in the grid configuration so that the protrusion having a first height is formed between adjacent pixel electrodes which are included in the different pixel electrode groups, and so that the protrusion having a second height, lower than the first height, is formed between adjacent pixels which are included in the same pixel electrode group.
In this embodiment, in the step of forming the protrusion, the protrusion having the first height (which is greater than the second height) is formed in the area between the pixel electrodes where a stronger transverse electric field takes place, thereby relatively intensifying the longitudinal electric field (namely, by shortening the gap between the edge of the pixel electrode and the counter electrode). On the other hand, the protrusion having the second height is formed in the area between the adjacent pixel electrodes where substantially no transverse electric field takes place, and it suffices to slightly intensify the longitudinal electric field.
In this embodiment, the method for manufacturing an electrooptical device may further include a step of forming an alignment layer on the plurality of pixel electrodes, and a step of subjecting the alignment layer to a rubbing process in a direction parallel to a step of the protrusion having the first height.
When the alignment layer deposited on the protrusion is subjected to the rubbing process in a direction parallel to the step of the protrusion having a relatively higher height, malfunction of the electrooptical material due to variations in a plan view in the thickness of the electrooptical material is controlled. Generally, if the rubbing process is performed in perpendicular to the step, the orientation of the electrooptical material is disturbed by the alignment layer that has undergone the rubbing process. The degree of disturbance increases with the magnitude of the step. By controlling the malfunction of the electrooptical material due to the variations in a plan view in the thickness of the electrooptical material in response to a large step (malfunction of the electrooptical material due to the variations in the thickness of the electrooptical material in response to a small step is essentially small regardless of the direction of the rubbing process), malfunction of the electrooptical material arising from the step of the protrusion is reduced.
In another embodiment of the manufacturing method of the electrooptical device of the present invention, the step of forming the protrusion may form striped protrusions in a plan view by forming the protrusion between the adjacent pixel electrodes which are included in the different pixel electrode groups, and form no protrusions between the adjacent pixel electrodes which are included in the same pixel electrode group.
In this embodiment, in the step of forming the protrusion, the protrusion is formed between the adjacent pixel electrodes included in the different pixel electrode groups (with the transverse electric field taking place), and no protrusion is formed between the adjacent pixel electrodes included in the same pixel electrodes (with almost no transverse electric field taking place). The striped protrusion thus formed in the area where the transverse electric field occurs reduces the adverse effect of the transverse electric field during the periodic polarity reversal driving.
In this embodiment, the method for manufacturing an electrooptical device may further include a step of forming an alignment layer on the plurality of pixel electrodes, and a step of subjecting the alignment layer to a rubbing process in a direction parallel to a step of the protrusion.
Malfunction of the electrooptical material arising from the step is controlled if the rubbing process is performed on the alignment layer formed on top of the protrusion in a parallel direction with the step of the protrusion. If the rubbing process is performed in a direction perpendicular to the step, the orientation of the electrooptical material is disturbed by the alignment layer that has been rubbed. By performing the rubbing process in a direction parallel with the step, malfunction of the electrooptical material attributed to the step is thus controlled.
In another embodiment of a method for manufacturing an electrooptical device, the step of forming the protrusion may form the protrusion using a wet etching process.
In accordance with this embodiment, the inclination of the step of the protrusion becomes moderate, and malfunction of the electrooptical material due to the step is controlled. Generally speaking, the steeper the angle of the step, the more the orientation of the electrooptical material is disturbed. With a moderate angle step formed through the wet etching process, the malfunction of the electrooptical material due to the step is reduced given the same height protrusion.
In yet another embodiment of a method for manufacturing an electrooptical device, the step of forming the protrusion may form the protrusion through a dry etching process and a wet etching process subsequent to the dry etching process.
In this embodiment, the protrusion is formed through the dry etching process with a high dimensional accuracy, and then the steep step protrusion formed through the dry etching process is moderated in the inclination thereof with the wet etching process subsequent to the dry etching process. A fine protrusion having a high position accuracy and a high dimensional accuracy is formed while the malfunction of the electrooptical material due to the step is controlled.
In still another embodiment of a method for manufacturing an electrooptical device, the step of forming the protrusion may include performing a photolithographic process which uses a mask that is used to produce the wiring during the step of forming the pattern.
In accordance with this embodiment, the same mask is shared by the photolithographic process for the formation of the wiring and by the photolithographic process for the formation of the protrusion. Manufacturing costs are thus low, compared with the case in which dedicated masks are used.
In this embodiment, the step of forming the protrusion may form the protrusion having a width different from the width of the wiring by using the mask and by adjusting an exposure level.
The same mask is shared by the photolithographic process for the formation of the wiring and by the photolithographic process for the formation of the protrusion, while the width of the wiring and the width of the protrusion are made different by adjusting the exposure level. This arrangement reduces the manufacturing cost while increasing designing variations.
In another embodiment, a method for manufacturing an electrooptical device may further include a step of forming the other protrusion on the second substrate facing the spacing between the adjacent pixel electrodes.
The protrusion arranged on the second substrate in this embodiment decreases the distance between the pixel electrode and the counter electrode in the area where the transverse electric field is generated, thereby relatively intensifying the longitudinal electric field. In addition, the presence of the protrusion weakens the transverse electric field, thereby reducing the effect of the transverse electric field on the electrooptical material. The adverse effect of the transverse electric field is reduced.
In this embodiment, the method for manufacturing an electrooptical device may further include a step of forming the other alignment layer on the second substrate, and a step of subjecting another alignment layer to a rubbing process in a direction parallel to a step of the other protrusion.
The malfunction of the electrooptical material due to the step is controlled by performing the rubbing process on the other alignment layer formed on the second substrate in a direction parallel to the step of the other protrusion.
The method for manufacturing an electrooptical device may further include a step of forming a light shield layer in a region on the second substrate facing the spacing between the adjacent pixel electrodes, wherein the step of forming the other alignment layer forms the other alignment layer using the presence of the light shield layer.
In this arrangement, the other protrusion is formed on the second substrate, using the light shield layer on the second substrate (the counter substrate), generally called black matrix or black mask (BM). The manufacturing process and the construction of the electrooptical device are advantageously simple, compared with the case in which dedicated layers are used in the formation of the protrusion.
To resolve this problem, an electrooptical device of the present invention includes a first substrate, a second substrate, and an electrooptical material interposed between the first and second substrates, wherein the first substrate includes a plurality of two-dimensionally arranged pixel electrodes, including pixel electrodes in a first group driven in a periodic polarity reversal manner with a first period and pixel electrodes in a second group driven in a periodic polarity reversal manner with a second period which is complementary to the first period, a pattern including a wiring that drives the pixel electrode and an element, and a protrusion that is formed in a spacing between pixel electrodes adjacent in a plan view, by performing a photolithographic process and an etching process onto a planarized top surface of the laminate of the first substrate after the planarization of the top surface of the laminate of the first substrate in a manufacturing process, and wherein the second substrate includes a counter electrode facing the plurality of pixel electrodes.
In the electrooptical device of this invention, the transverse electric field takes place between the adjacent pixel electrodes belonging to the different pixel electrode groups (i.e., the adjacent pixel electrodes supplied with opposite polarity voltages). Through the etching process, the protrusion is positively formed in the edge of each pixel electrode adjacent to or in the non-aperture area of each pixel. First, the longitudinal electric field taking place between the pixel electrode and the counter electrode is relatively intensified with respect to the transverse electric field taking place between the adjacent pixel electrodes if the edge of each pixel electrode is positioned on the top of the protrusion. Second, regardless of whether the edge of the pixel electrode is positioned on the top of the protrusion, the transverse electric field taking place between the adjacent pixel electrodes is weakened by the dielectric constant of the protrusion, and further, the influence of the transverse electric field on the electrooptical material is reduced by reducing the volume of the electrooptical material through which the transverse electric field passes. The electrooptical device is thus free from malfunction of the electrooptical material such as the orientation defect of the liquid crystal arising from the transverse electric field caused in any of the alternating drive methods. In this case, the edge of the pixel electrode may reach the top of the protrusion as discussed above, or may not reach the top of the protrusion. Alternatively, the edge of the pixel electrode may extend to a midway point on the slant wall of the protrusion or on the substantially vertically upright wall of the protrusion.
The central portion of the pixel electrode in the aperture area of each pixel is formed on the positively planarized surface. This arrangement reduces malfunction of the electrooptical material such as the orientation defect of the liquid crystal arising from variations in the thickness of the electrooptical material encapsulated between the pixel electrode and the counter electrode. Further, since a light shield layer that covers the malfunctioning point of the electrooptical material is reduced in size, visible defects such as unlit defects are prevented, and the aperture ratio of each pixel electrode is increased.
As a result, malfunction due to the transverse electric field in the electrooptical material such as a liquid crystal is substantially reduced, and an electrooptical device such as a liquid-crystal device presents a high-contrast, bright, and high-quality image.
The present invention is applicable to not only a transmissive-type electrooptical device and a reflective-type electroopitcal device, but also other variety of electrooptical devices.
These operations and other advantages will become obvious from the following discussion of the embodiments of the present invention.